You are a PA at a GP practice and you are seeing a four-year-old boy with his mum. She is concerned about his persistent cough, which is productive, his wheezing, shortness of breath, poor growth despite his good appetite and his greasy stools that are difficult to flush. He has also had three chest infections in the past 6 months and is also complaining that his skin always tastes salty. What is the most appropriate test to confirm the diagnosis of this condition?